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A066230
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f-perfect numbers defined by f(n) = n - 1 (where f-perfect numbers are defined in A066218).
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4
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1, 12, 196, 368, 1696, 30848, 437745, 2075648, 8341504, 33452032, 34355150848
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OFFSET
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1,2
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COMMENTS
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Equivalently, let g(n) = sigma(n)-n-d(n)+2, where d(n) is the number of divisors of n and sigma(n) is their sum. Then n is in the sequence if g(n)=n.
It seems that if 2^(i+1)-(2*i+1) is prime, 2^i*(2^(i+1)-(2*i+1)) is in the list. For example i in {2, 4, 5, 7, 10, 11, 12, 17, 24, 30, 34, 113, 151, 185}. No other exceptions than 1, 196 and 437745 for n < 10^8. - Lambert Klasen (lambert.klasen(AT)gmx.net), Jul 31 2005
If 2^(i + 1)-(2i + 1) is prime then n = 2^i*(2^(i + 1)-(2i + 1)) is in the sequence because sigma(n)-d(n) + 2 = (2^(i + 1)-1)*(2^(i + 1)-2i)-2(i + 1) + 2 = 2^(i + 1)*(2^(i + 1)-(2i + 1)) = 2n, so sigma(n)-n-d(n) + 2 = n. -Farideh Firoozbakht, Sep 18 2006
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LINKS
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EXAMPLE
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f(12) = 11 = 0 + 1 + 2 + 3 + 5 = f(1) + f(2) + f(3) + f(4) + f(6), hence 12 is a term of the sequence.
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MATHEMATICA
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g[ n_ ] := DivisorSigma[ 1, n ]-n-DivisorSigma[ 0, n ]+2; For[ n=1, True, n++, If[ g[ n ]==n, Print[ n ] ] ]
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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